1. Field of the Invention
The present invention relates to a high voltage power supply control apparatus used in an electrophotographic mechanism section having a photosensitive body in a copying machine, a laser printer, or the like.
2. Description of the Related Art
An electrophotographic mechanism section used in a copying machine, a laser printer, or the like comprises, e.g., a photosensitive drum. After a photosensitive body of the photosensitive drum is charged by a charging unit, the photosensitive body is exposed by an exposure unit to form an electrostatic latent image on the photosensitive body. A toner is applied to the electrostatic latent image by a developer unit to form a toner image. The toner image is transferred onto transfer paper by a transfer unit.
The electrophotographic mechanism section comprises a high voltage power supply controlling apparatus for applying a high voltage to a charger in the charging unit, a developer sleeve in the developer unit, and a transfer charger in the transfer unit.
In this apparatus, however, since PWM (pulse-width modulation) controllers and A/D (analog-to digital) converters are arranged in correspondence with charging, transferring, and biasing sections, the number of circuit members to be used is undesirably increased, and the apparatus becomes bulky. Since the PWM controllers are independently operated, each PWM controller suffers from a long operation rest period, resulting in poor operation efficiency.
The applicant of the present invention has proposed an improved high voltage power supply control apparatus in which a set of a PWM controller and an A/D converter is commonly used for a plurality of high voltage generating controllers so as to decrease necessary circuit members as well as to reduce the size of the apparatus, thereby increasing the operational efficiency of the PWM controller. (Such an apparatus is disclosed in Japanese patent application No. H 2-197852 which corresponds to U.S. patent application Ser. No. 07/735,300 filed on Jul. 24, 1991, now U.S. Pat. No. 5,202,725, issued Apr. 13, 1993.)
The above apparatus basically performs a comparison between a given reference value and a digital value obtained from the A/D converter so as to add the difference between them to the feedback value or to subtract such difference from the feedback value, so that a duty ratio for controlling the switching operation of switching means varies.
Meanwhile, a monitor voltage of a high voltage power supply controller is liable to be subjected to a noise. If such a noise is contained to the monitor voltage, the magnitude of the difference between the reference value and the digital value obtained by A/D-converting the monitor voltage temporally increases, so that the duty ratio for controlling the switching operation of the switching means changes by a large amount. Then a new problem arises in that a high voltage applied from a high voltage transformer to a load largely deviates from a normal value.